To determine how much is some percentage of a given amount, we divide this amount by 100 and then we multiply it by the number of the percentage we are looking for.
If Robert decreased his weight by 10%, it means that he weighs 90% of the original value. And to find 90% of his weight, we need to divide it by 100 and multiply by 90, as follows:
[tex]\begin{gathered} W_{new}=W_{old}\cdot\frac{90}{100}=180\cdot\frac{90}{100}=1.8\cdot90 \\ \\ W_{new}=162\text{ pounds} \end{gathered}[/tex]Robert weighs 162 pounds.
